Impurities such as sulfur in diesel fuels require removal, typically by hydrotreating, in order to comply with product specifications and to ensure compliance with environmental regulations. For example, beginning with the 2007 model year, pollution from heavy-duty highway vehicles was required to be reduced by more than 90 percent. Sulfur in diesel fuel was required to be lowered to enable modern pollution-control technology to be effective on such heavy-duty highway vehicles as trucks and buses. The United States Environmental Protection Agency required a 97 percent reduction in the sulfur content of highway diesel fuel from a level of 500 ppm (low sulfur diesel, or LSD) to 15 ppm (ultra-low sulfur diesel, or ULSD). These new regulations required engine manufactures to meet the 2007 emission standards and to have the flexibility of meeting the new standards through a phase-in approach between 2007 and 2010. These standards are comparable to those in most industrialized nations.
Some of the processes presently in commercial use for producing diesel fuels will not be capable of sufficiently reducing the sulfur content to the new required levels without modifications of some existing hydrotreating processes and equipment. Hydrotreating is an established refinery process for improving the qualities of various petroleum streams from naphtha boiling range streams to heavy oil boiling range streams. Hydrotreating is used to remove contaminants, such as sulfur, nitrogen and metals, as well as to saturate olefins and aromatics to produce a relatively clean product stream for downstream product sales.
Diesel fuels are typically hydrotreated by passing the feed over a hydrotreating catalyst at elevated temperatures and pressures in a hydrogen-containing atmosphere. One suitable family of catalysts that has been widely used for this service is a combination of a Group VIII metal and a Group VI metal of the Periodic Table, such as cobalt and molybdenum, on a support such as alumina. After hydrotreating, the resulting product stream is typically sent to separator to separate hydrogen sulfide and light gases from the treated stream. The resulting hydrotreated stream can then be sent to a stripper to produce two or more desired fractions, such as a diesel fuel fraction and a wild naphtha fraction.
A substantial portion of the diesel pool must now have to comprise ultra-low sulfur diesel. This is putting a great deal of pressure on refiners to find ways to meet the growing demand for such ultra low sulfur feedstocks. Low pressure distillate hydrotreating process unit have been used for many years for removing sulfur from distillate feeds. Low pressure distillate hydrotreating units were the norm until recently because they were able to meet the sulfur requirements at the time. As sulfur requirements became more and more stringent, higher pressure units were needed. In many instances, grass root high pressure distillate hydrotreating process units were built and in other instances older lower pressure units were totally dismantled and replaced with new higher pressure units. Completely replacing a lower pressure hydrotreating process unit with a higher pressure unit, or building a grass roots unit, is very expensive. Therefore, there is a need in the art for ways to revamp existing lower pressure hydrotreating units to higher pressure hydrotreating units at substantially less cost than completely scraping the lower pressure units and replacing it with grass roots high pressure units.